Organic LEDs (OLEDs) have long been touted as the exclusive materials for flexible optoelectronic/electronic devices. Some familiar examples of daily use of flexible OLED display products include smart phones and flat screen LED televisions. However, OLEDs have drawbacks, such as a short lifetime (˜10,000 hrs), low efficiency, and low stability in high humidity conditions compared to inorganic LEDs. That is, inorganic emitters are superior to OLEDs in many aspects, including stability, output power, efficiency, and lifetime. Also, inorganic LEDs have a wide spectral range, from near-ultraviolet to infrared, and the emission wavelength can be tailored and designed based on the amount of indium in the composition and/or the thickness of the quantum wells in the active region of the inorganic LED.
Typically, inorganic LEDs are grown on thick, hard, stiff substrates, including but not limited to sapphire, silicon, silicon carbide or gallium nitride, using the growth techniques of metal oxide chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). As such, these devices are not able to be deformed or bent without damage to the mechanical structure.
However, recent research efforts have brought Group III-nitride light-emitting diodes (LEDs) on a flexible platform closer to reality. The Group III-nitride inorganic devices, in particular gallium nitride (GaN) devices, have been used commercially in green, blue, and white LEDs, and have proven long lifetimes with high brightness and efficiencies. Existing demonstrations of flexible inorganic LEDs rely on the transfer of micro-patterned LED arrays from sapphire or Si substrates onto a plastic/polymer foreign substrate, so that the emitters appear to adopt the curvilinear profile of the flexible substrate. However, the LEDs themselves are not flexible; the flexibility is still limited to the extrinsic substrate.
Chun et al. (“Transfer of GaN LEDs from Sapphire to Flexible Substrates by Laser Lift-Off and Contact Printing, ” IEEE Photon. Technol. Lett. 24, 2115, 2012) disclose fabrication of flexible GaN-based LED systems by laser lift-off (LLO) and transfer printing methods. LLO enables transferring a whole GaN LED layer from sapphire onto a silicon handling wafer to provide a stable platform for any shape of LED. Polymer pedestal structures underneath the LEDs support efficient transfer printing of the patterned LED array from the silicon handling wafer to a flexible substrate.
However, there are significant drawbacks associated with the method of placing inorganic LEDs on silicon wafers, including non-uniformities between individual emitters, complex assembly processes, high manufacturing cost, and poor reliability. More importantly, the LEDs themselves are not flexible; the flexibility is still limited to the extrinsic substrates.